This invention relates to a device for providing a pre-intrusion signal, particularly of the type which warns when an intruder is trying to open a door to gain entry. It is primarily designed to be associated with a doorknob, and to detect the proximity of an intruder's hand to the doorknob. The detected signal can then sound a warning alarm, or actuate any suitable protective device.
More specifically, I have invented a simple, highly practicable, battery-operated electrical device which can be hung on the inside of the doorknob, and which is fully self-contained, i.e., it requires no electrical connection to either an electrical power source or an external ground.
Many devices of this general type have been proposed, but the problems inherent in such detection devices have not heretofore been satisfactorily solved. Such devices, as a practical matter, do not provide an adequate intrusion signal unless they are externally connected, thereby losing the benefits and convenience of a self-contained unit.
The prior art devices intended to solve the problem have fallen into two general categories: (1) "Passive" devices--which are arranged to pick up electromagnetic or electrostatic fields generated externally, and which respond to the additional antenna effect created by an intruder; or (2) "Ground Capacitance" devices--which utilize oscillator circuits, and which experience altered circuit values when an intruder establishes an external capacitance relative to an external earth ground. Of the two prior art types, the "ground capacitance" devices appear to be more numerous.
The prior art devices of the ground capacitance type are represented by the following U.S. Pat. Nos. Bagno 3,199,096; Fontaine 3,623,063; Domin et al 3,697,971; Gehman 3,706,982; Atkins 3,735,379; Guetersloh 3,829,850; Bolle et al 4,021,679; and Tanaka et al 4,030,037. The intrusion-detection systems of each of the listed patents share certain attributes. They each rely on an external ground capacitive effect which occurs when the intruder is physically coupled to the oscillator. Also they each use changes resulting from such external ground capacitive effect to alter internal circuit values, such as oscillator output levels or frequencies, thereby causing an output signal. In such systems, the human capacitance represents a capacitive loading on the oscillator.
The prior art devices of the passive type are represented by U.S. Pat. Nos. Dettling et al 3,771,152 and Geiszler et al 3,956,743. A device of this type relies on the "antenna effect" of an intruder in causing a change in the received signal from an electromagnetic or electrostatic field. The detector signal increases because the intruder constitutes, in effect, an extension of the antenna, which is receiving "passively" the pickup from the field. In other words, the device functions by detecting the change in the charge on the antenna.
Both of the types of intrusion detectors discussed above have serious functional problems. They can operate satisfactorily if they are externally connected, or grounded, e.g., if they are plugged into an available electrical system. But it is important, as a practical matter, that the detector unit be self-contained.
When such prior art detectors are self-contained, they inherently have very weak signal changes to respond to. This is true because, whether the intruder forms an antenna or a capacitor, the human effect represents a very small addition to the existing antenna or to the existing capacitance. This is particularly serious in view of the wide range of doorknobs, metallic door frames and metallic ornametation commonly associated with door openings. The change in signal level caused by the intruder is relatively small, substantially less than 5% of the total signal level, and it is very difficult to detect reliably.
Where the device is battery-operated, the reference to earth ground is substantially non-existent, and can only be described as a current leakage, thus increasing the problem of small signal change detection. In other words, the grounding required to complete the circuit, of which the intruder forms a part, exists only to the extent of leakage; and the intrusion signal is thus minimized by the high impedance of that circuit.
As discussed in several of the prior art patents, noise is a significant problem in units which function by using a change in capacitance to vary the frequency or amplitude of an oscillator. The extraneous causes of signal changes, such as temperature change, humidity change, household 60-cycle current, etc., can cause spurious detector responses. This results in part from the weakness of the detected intrusion signal, and in part from the fact that the presence of noise causes the oscillator to change frequency or amplitude.
In the light of the deficiencies in prior art devices, and after extensive experimental efforts, I have concluded that, in order to have a successful battery-operated device for detection of a human contact with, or proximity to, a doorknob, it is necessary to generate and detect a strong signal by relying on a ground system within the detection instrument itself, thereby avoiding the necessity of working with a very small signal limited to a leakage path to earth ground.